Chlorofluorocarbons (CFC) or hydrochlorofluorocarbons (HCFC) were widely employed in the field of low and very low temperature. Thus, R-22 (chlorodifluoromethane) and R-502 (azeotrope of chlorodifluoromethane and chloropentafluoroethane) with single- or two-stage systems made it possible to reach temperatures ranging down to -35.degree. C., while R-13 (chlorotrifluoromethane) and R-503 (azeotrope of trifluoromethane and chlorotrifluoromethane) with cascade systems (with two compressors) made it possible to go down to -80.degree. C.
In these cases, by virtue of thermodynamic properties close to those of the CFCs, hydrofluorocarbons (HFC) such as HFC-23 (trifluoromethane), HFC-116 (hexafluoroethane), HFC-32 (difluoromethane), HFC-125 (pentafluoroethane), HFC-143a (1,1,1-trifluoroethane) and HFC-134a (1,1,1,2-tetrafluoroethane), pure or mixed, have enabled substitutes to be developed.
However, a fluid had been developed in the past for making the transition between the above two configurations. This is a bromofluorocarbon (BFC), namely bromotrifluoromethane (R-13B1) which made it possible, with a single compressor, to obtain temperatures of -60.degree. C. with a single- or two-stage system.
Like the CFCs, R-13B1 has been subjected to rules by the Montreal Protocol because of its large effect on ozone, and its production has been prohibited since Jan. 1, 1996.
Among the HFCs available at present none exhibits properties close to those of R-13B1, as the table below shows. In particular, none has a boiling point close to that of R-13B1, which would make it possible to go down to temperatures close to -60.degree. C.:
TABLE 1 ______________________________________ Vapour Liquid Boiling Critical .DELTA.Hv at d d Pressure T T 1 atm at 1 atm at 25.degree. C. at 25.degree. C. (.degree. C.) (.degree. C.) (kJ/kg) (kg/m.sup.3) (kg/dm.sup.3) (bar) ______________________________________ R-13B1 -57.7 67 119 8.72 1.538 16.2 HFC-23 -82.1 25.9 238.6 4.68 -- -- HFC-116 -78.3 19.7 117.7 9.00 -- -- HFC-32 -51.7 78.4 381.5 2.98 0.961 16.9 HFC-125 -48.1 66.3 169.7 6.73 1.189 13.8 HFC-143a -47.4 72.9 220.2 4.88 0.927 12.6 HFC-134a -26.4 101.1 215.9 5.28 1.206 6.7 HFC-218 -36.7 71.9 104.4 10.25 1.323 8.8 HFC-290 -41.6 96.7 427.8 2.40 0.494 9.3 ______________________________________
HFC-116 is the fluid nearest to R-13B1, but its critical point, which is too low (&lt;20.degree. C.), does not allow it to be employed in mono-stage systems with a condensation temperature higher than 30.degree. C.
In the case of the other HFCs, which have higher critical temperatures, their thermodynamic properties and, in some cases, their flammability mean that no pure substance can replace R-13B1.